I'd pretty much given up, therefore my congratulations have to go to Ross Dargahi (found via Juston Blanton) for working it out...
The instructions below works for my Orange SPV M5000 (a.k.a the HTC Universal) and my Apple Powerbook, running Mac OS X 10.4.4. Your milage may vary, but from other reports they seem to be fairly generic for all Windows Mobile 5.0 devices.
- Download Ross Barkman's iPAQ modem script, and install this into /Library/Modem Scripts. It's okay to rename this file to something more appropriate, e.g. HTC Universal GSM, if you want to do that.
- On your HTC Universal enable Receive all incoming beams in Settings > Beam, and then turn on Bluetooth amnd make the device discoverable in Settings > Bluetooth.
- Set up an new Incoming Port (COM0) under Settings > Bluetooth > COM Ports.
- If you're running Mac OS X 10.4 then go to System Preferences > Bluetooth > Set Up New Device to run the Bluetooth Setup Assistant. When prompted choose Any Device when asked to select the device type, as the HTC Universal does not advertise itself correctly as a phone. If you're running 10.3.9 then you should see Ross Barkman's comments on my post talking about my problems with Panther and the Universal's dial-up networking stack.
- Select your device from the list of detected Bluetooth devices and pair the devices in the normal way, remembering to select Access the Internet with your phone's data connection when prompted.
- Enter *99# into the Phone number field, rather than your networks APN. For instance I'm on Orange in the UK, but despite this I still enter *99# rather than the Orange APN, "orangeinternet" into the field. Additionally, even if your network does not require a username and password you should enter something in those fields. Orange doesn't require me to pass a username or password, but none the less I entered "orange" for both. However if your network doesn't require it, you can enter whatever string you like here it won't make a difference. But if you don't do this, the script will not work correctly. Information about APN information for your network can be found either form your network provider, or from Ross Barkman's GPRS information page.
- Select your freshly installed modem script in the drop down menu in the setup assistant and finish setting up your device.
- In System Preferences > Network > Bluetooth, in the PPP tab click on PPP Options and make sure that Use TCP header compression is unchecked.
- Go to the Bluetooth Modem tab and make sure that Use TCP header compression is unchecked. Disable both Enable error correction and compression in modem and Wait for dial tone before dialing and apply your changes.